1. Field of the Invention
This invention relates to a photosensitive member to be used for an image-forming apparatus and also to an image-forming apparatus comprising such a photosensitive member as well as to an image forming process. More particularly, it relates to a photosensitive member to be used for an image-forming apparatus adapted to electrostatically charge the surface of its photosensitive member operating as image carrier, writing video information on the electrostatically charged surface by means of a beam of visible light or a linear-scanning laser beam and forming an image typically by means of toner and comprising a cleaning means for clearing the surface potential of the photosensitive member after transferring the toner image. It also relates to an image-forming apparatus comprising such a photosensitive member as well as to an image forming method using such a photosensitive member.
More specifically, the present invention relates to a photosensitive member to be used for an image-forming apparatus having specific surface characteristics including, in particular, the wettability (W) of the photosensitive member relative to the adherers of the surface thereof drawn out from the amount of surface free energy (.gamma.) to thereby control the foreign objects such as toner adhering to the surface and make it apt to form images with a good image quality for a prolonged period of time regardless of fluctuations of environmental factors including moisture and temperature. It also relates to an image-forming apparatus comprising such a photosensitive member as well as to an image forming method using such a photosensitive member.
2. Related Background Art
Currently available image-forming apparatus, electrophotographic apparatus in particular, include printers operating as output means of computers and word processors that have been finding an ever-increasing demand in recent years, as well as copying machines. Since such appratus are operated in a variety of operating environments, they are more often than not provided with means for stabilizing the output image such as means for eliminating the influence of fluctuations of environmental factors on the density of the output image. In addition, such printers are required to be low cost and maintenance free particularly because they are used not only for office applications but also for home or personal applications.
Still additionally, such printers are required to be friendly to the environment from the ecological point of view and hence should be adapted to print on the opposite surfaces of a sheet of copy paper, which may be recycled paper, and reduce the consumption rate of paper and electric power.
FIG. 1 of the accompanying drawings is a schematic block diagram of an image-forming apparatus, illustrating the image-forming process of a copying machine.
Referring to FIG. 1, reference numeral 101 denotes a photosensitive member of the image-forming apparatus to be used with an electrophotographic system (hereinafter simply referred to as "photosensitive member"), which is surrounded by a principal corona unit 102, an electrostatic latent image forming site 103, a developing unit 104, a copy paper feeding system 105, a transfer corona unit 106a, a separation corona unit 106b, a cleaner unit 107, a delivery system 108 and a conditioning light source 109 arranged clockwise in FIG. 1. If necessary, the photosensitive member 101 may be provided with a circumferential internal surface heater 125 for controlling the temperature of the photosensitive member 101.
The surface of the photosensitive member 101 is uniformly and electrostatically charged by the principal corona unit 102 and, in operation, exposed to light at the electrostatic latent image forming site 103 to form an electrostatic latent image thereon.
The electrostatic latent image is then turned into a visible toner image by the developing sleeve of the developing unit 104 that carries toner on the surface.
Meanwhile, copy paper P is fed from the copy paper feeding system 105 as it is guided by a copy paper guide 119 and its leading edge is registered by register rollers so that the toner image formed on the surface of the photosensitive member 101 is transferred onto the copy paper P by means of the transfer corona unit 106a. Then, the copy paper P is separated from the photosensitive member 101 by means of the separation corona unit 106b and/or a separation means such as a separation pawl (not shown) and subsequently the toner image on the surface of the paper is moved to a fixing unit 123 by means of the delivery system 108, where the toner image is fixed by fixing rollers 124 arranged in the fixing unit 123 before it is delivered to the outside of the image-forming apparatus.
On the other hand, after the toner image is transferred to the copy paper P, the surface of the photosensitive member 101 is cleaned by a cleaning blade 120 and a cleaning roller (or brush) 121 arranged in the cleaning unit 107 to remove the residual toner and the fine particles of paper adhering to the surface in order to make it ready for the next copying cycle.
As described above, an image-forming apparatus adapted to repeat the cycle of operation of forming a toner image on the surface of a photosensitive member and transferring the toner image onto a copying material such as copy paper needs to be provided with a cleaning means for removing the foreign objects remaining on the surface of the photosensitive member including the residual toner after transferring the toner image.
Such a cleaning unit 107 typically comprises a cleaning blade made of rubber or resin and a cleaning brush made of resin fiber. The powdery magnetic objects remaining on the surface of the photosensitive member such as the residual toner may alternatively be removed by means of magnetic adsorption.
Now, such a cleaning unit and cleaning means that can be used for the unit will be described below by referring to FIG. 2 of the accompanying drawings.
FIG. 2 is a schematic view of a cleaning unit that can be used for the image-forming apparatus of FIG. 1.
Cleaning means that can be used for the cleaning unit 301 of FIG. 2 may comprise a cleaning blade 302 made of urethane rubber, a cleaning roller 303 made of silicon rubber, sponge or a magnetic material, a doctor roller 304, a waste toner pool 305 and a waste toner delivery system 306.
The doctor roller 304 may be arranged whenever necessary and show a blade-like shape. Then, it will be referred to as scraper (or doctor blade).
For the purpose of simplification, the scraper will be omitted from the following description of the components of the cleaning unit.
Referring to FIG. 2, reference numeral 301 denotes a cleaning unit comprising a cleaning blade 302 made of a material obtained by mixing urethane rubber and one or more than one silicon compounds to make it show appropriate elasticity and hardness.
A cleaning roller 303 made of a magnet is arranged at an upstream position (lower position in FIG. 2) relative to the cleaning blade 302 in the sense of rotation of the photosensitive member. The cleaning roller 303 attracts powdery magnetic materials including the toner by its magnetic force and hence comes to be coated with the adherers. Thus, the coat of the powdery magnetic materials abuts the surface of the photosensitive member with an appropriate abutting width (referred to as "nipping width") and is made to scrub the surface of the photosensitive member at a predetermined relative speed.
While the cleaning roller 303 is made of a magnet in the above description, it may alternatively be a roller that is biased with the polarity opposite to that of the toner or made of silicon rubber of spongy resin.
Still alternatively, the cleaning roller 303 may be replaced by a brush-shaped member made of a material selected appropriately by taking the hardness of the photosensitive member and the processing speed of the image-forming apparatus.
When the brush is used with a photosensitive member showing a high surface hardness such as an a-Si type photosensitive member, it may be a chemical fiber brush made of polyethylene or polystyrene or a brush made of electroconductive fiber obtained by adding carbon to chemical fiber in order to provide the fiber with an desired level of electroconductivity or fiber of amorphous metal (e.g., "BOLFUR": tradename, available from Unitika).
The nipping width of the photosensitive member 101 and the cleaning roller or the cleaning brush is desirably held to a constant value in order to realize a constant cleaning performance and prevent any problem such as an abraded photosensitive member due to excessive local abutment from occurring.
The mechanism for holding the cleaning roller or the cleaning brush in abutment with the photosensitive member 101 may be realized by using small rollers held in abutment with the photosensitive member in an area other than the image-forming site or by pushing the roller against the photosensitive member under a predetermined level of pressure. In the case of a cleaning roller made of a magnetic material, a constant nipping width can be realized by regulating the thickness of the toner coat.
The cleaning unit may also be realized by removing part of the above components or using one or more than one additional components.
FIGS. 3A through 3D of the accompanying drawings illustrate how a cleaning operation is repeated for an image-forming apparatus of the type under consideration.
Now, the cleaning operation will be described by referring to FIGS. 3A through 3D. Note that the photosensitive member 101 is made to show a straight surface (with no radius of curvature) for the purpose of simplicity.
Step 1
As the photosensitive member 101 with which the cleaning unit 301 is held in abutment is driven to rotate at a predetermined rate of revolution. In the step of operation of FIG. 3A, the surface of the photosensitive member 101 moves from left to right to come closer to the cleaning blade 302.
The photosensitive member 101 carries on the surface thereof a toner image formed by said steps of electrostatically charging the surface, forming a latent image thereon and developing the latent image.
The adherers 3001 including the toner that has not been transferred to the copying paper and pieces of rosin and talc are also brought closer to the cleaning unit as they are forced to adhere to the surface of the photosensitive member by electrostatic force, inter-molecular force, frictional force and other force that makes them adherent.
If necessary, the photosensitive member is held to a predetermined temperature level.
As described above, the cleaning unit may not comprise a cleaning roller 303 (or a cleaning brush, which will not specifically be mentioned hereinafter).
When the cleaning blade 302 is used at the site of abutment with the surface of photosensitive member, powder may often be applied to it to provide a lubricating effect. In the step of FIG. 3A, part of the collected waste toner or the toner held to the cleaning roller by an appropriate means is appropriately supplied for use from the cleaning roller 303 by way of the toner pool 307.
Step 2
If the cleaning unit comprises a cleaning roller 303, the above described adherers 3001 including the residual toner are scrubbed and scraped or sucked by the cleaning roller 303 for collection. The adherers 3001 are then taken up into the cleaning roller 303 (FIG. 3B).
Step 3
The adherers 3001 that include the residual toner and are taken up by the cleaning roller 303 are then partly collected by an appropriate mechanism such as a doctor roller 304 (or a doctor blade, which will not specifically be mentioned hereinafter). The collected adherers 3001 including the residual toner are then fed to the toner pool 305 within the cleaning unit 301 (FIG. 3C).
As described above, the residual toner may be discharged from the cleaning roller 303 at an appropriate rate from the viewpoint of lubricating effect of the cleaning blade 302 on the photosensitive member.
The collected toner is then moved into a waste toner container (not shown) by way of the waste toner delivery system 306.
Alternatively, the collected toner may be screened and the screened toner may be partly or mostly reused.
Step 4
The adherers 3001 including the residual toner not collected by the cleaning roller 303, the residual toner in case of a system not comprising a cleaning roller 303 or the residual toner left after the discharge of toner from the cleaning roller are brought closer to the cleaning blade 302 as they remain adhering to the surface of the photosensitive member 101. Then, the residual toner and other adherers are scraped off and collected typically by the cleaning blade 302 of the cleaning unit 301.
The collected toner is then moved to a waste toner storage container (not shown) by way of the waste toner delivery system 306 comprising a screw and delivered further away (FIG. 3D).
The waste toner storage container may be arranged at a position (not shown) within the image-forming apparatus or, alternatively, incorporated in the cleaning unit when the image-forming apparatus is a cartridge type laser beam printer (LBP).
The electrostatic latent image that is left on the surface of the photosensitive member is erased by a conditioning light source 109 (see FIG. 1).
As described above, the cleaning roller 303 may be replaced by a cleaning brush that is held in abutment with the surface of the photosensitive member to scrape off various adherers from the latter.
As alternative, there has been proposed the use of a magnetic cleaning roller made of a magnetic material, a cleaning roller biased with the polarity opposite to that of the toner or a cleaning roller made to show properties opposite to those of the toner, which is made to collect the residual toner on the surface of the photosensitive member in a non-contact way or as it is brought to contact directly with the surface of the photosensitive member or indirectly therewith by way of the toner already sucked by and deposited onto the surface thereof.
Such devices (cleaning blade, cleaning brush, cleaning roller, etc.) are selectively arranged within the cleaning unit and used independently or in combination so as to effectively remove foreign objects and powder of the toner from the surface of the photosensitive member.
As pointed out earlier, an increasing number of image-forming apparatus are being used under various different operating conditions including a well air-conditioned environment and extending between a low temperature/light moisture setting and a high temperature/heavy moisture setting.
In view of the use in a particularly harsh environment, there is a strong demand for image-forming apparatus that operate electrophotographically stably without giving rise to problems such as a poor cleaning performance and adhesion of molten toner so as to make them meet the requirement of maintenance free and a long service life.
Thus, image-forming apparatus using an electrophotography system are required to stably provide clear and high quality images for a prolong period of time regardless of environmental variations as they find more and more personal applications with diversified operating environment. Additionally, they have to meet the requirement of down-sizing and cost reduction.
In order for an image-forming apparatus to provide clear and high quality images for a prolonged period of time, then it is necessary to precisely control the latent image and uniformly clean the surface of the photosensitive member. Additionally, the cleaning unit of the image-forming apparatus has to be down-sized and comprise a reduced number of components that are simply configured.
However, as the cleaning system is simplified and made to show a long service life, there arises a problem that the residual toner is, if partly, not removed by the cleaning blade 302 and other members and remains on the surface of the photosensitive member.
The remaining adherers will then be subjected to the steps from the electrostatically charging step on for more than once.
Additionally, the adherers remaining on the surface of the photosensitive member can be spread over a wider area of and/or laid higher from the surface of the photosensitive member as they are scraped by the cleaning blade 302 and the cleaning brush or the cleaning roller 303 and also by the copying material (not shown) and/or the heat existing on the surface.
Furthermore, as the above steps are repeated, additional foreign objects may adhere to the surface to increase the area and the height of the adherers.
Thus, the adherers that are not removed from the surface of the photosensitive member by the cleaning unit gradually grow until they eventually become visually recognizable black spots on the images produced by the apparatus.
Particularly, if the image-forming apparatus is used after a long pause, the toner and the debris of paper collected in the cleaning unit (hereinafter referred to collectively as the collected toner) are often found to have agglomerated within the unit.
If the collected toner is not found to have agglomerated when the apparatus is used after a long pause, the residual toner located near the contact point or line of the surface of the photosensitive member and the cleaning unit and the collected toner can often become agglomerated as the temperature rises near the photosensitive member of the apparatus to consequently raise the temperature of the toner.
Particularly, in an image-forming apparatus provided with a heater for regulating the surface temperature of the photosensitive member, the toner found on the surface of the photosensitive member and the cleaning unit can become agglomerated to give rise to a phenomenon referred to as blocking phenomenon that damages the cleaning means of the cleaning unit including the cleaning blade and the cleaning roller by the temperature rise in the initial stages of the image-forming operation conducted after a long pause.
Additionally, as the adhering toner grows, there arise a number of problems to the cleaning unit such as damaged cleaning members including a chipped or burred cleaning blade and a cleaning roller having one or more than one grooves formed on the surface, a vibrating cleaning blade and an uneven nipping width extending between the cleaning roller and the photosensitive member and along the axis of the photosensitive member. Such problems can give rise to an abnormally cleaned condition on the part of the surface of the photosensitive member.
Then, the surface of the photosensitive member shows "defective cleaning", which is far from a satisfactorily cleaned state.
Defective cleaning by turn can give rise to disadvantageous phenomena such as "black streaks" of toner produced by a chipped cleaning blade, "filming" that makes the entire surface of the photosensitive member thinly coated with toner and "fusion" of toner that produces black spots on the image.
Additionally, both the coat of toner on the surface of the cleaning roller and the pressure of the cleaning roller applied to the photosensitive member can show local unevenness to make the surface of the photosensitive member become scraped unevenly.
Then, rays of light striking the photosensitive member can be refracted unevenly to give rise to interference, which by turn produces local variations in the effective quantity of light entering the photoconductive layer of the photosensitive member and hence an uneven image density.
These and other phenomena degrade the quality of image and make some of the components of the photosensitive member and the cleaning unit require frequent servicing and even replacement so that the image-forming apparatus as a whole becomes far from maintenance free.
Various techniques have been proposed and are currently used in order to eliminate such problems by completely removing the foreign objects adhering to the surface of the photosensitive member. Known techniques include the following:
(1) a technique of controlling the pressure (abutment pressure) under which the cleaning member such as the cleaning blade, the cleaning brush or the cleaning roller is made to abut the photosensitive member; PA1 (2) a technique of selecting an optimal relative speed of the cleaning member and the photosensitive member and using an optimal material for the cleaning member to improve the effect of scraping the adherers; PA1 (3) a technique of modifying the surface profile of the cleaning roller typically by forming a helical groove on the surface; and PA1 (4) a technique of controlling the cleaning operation by means of a magnetic material or a bias.
A phenomenon of "smeared image (caused by heavy moisture)" that occurs when the image-forming apparatus is used in a heavy moisture/high temperature environment can get to be definitely apparent as the surface of the photosensitive member becomes apt to adsorb moisture under the influence of corona products attributable to ozone that is produced from the corona unit as the latter is used repeatedly. Then, the phenomenon by turn gives rise to a lateral flow out of the electrostatic charge and a smeared image.
In the case of an a-Si type photosensitive member, Japanese Utility Model Publication No. 1-34205 describes an anti-smeared image measure using a heater to drive off the moisture that has been adsorbed by the surface of the photosensitive member. Similarly, Japanese Patent Publication No. 2-38956 describes a method of removing corona products from the surface of the photosensitive member by brushing the surface by means of a brush formed from a magnetic roller and a magnetic toner. Japanese Patent Application Laid-Open No. 61-100780 describes a method of removing corona products by scrubbing the surface of the photosensitive member by means of an elastic roller.
On the other hand, a cleaning roller or a cleaning brush as described above may also be used to scrub the surface of the photosensitive member.
A technique of scrubbing the surface of the photosensitive member is particularly feasible when the surface is very hard as in the case of an a-Si type photosensitive member.
In the case of a relatively soft photosensitive member such as an organic photosensitive member (OPC), there have been proposed a technique of designing an electrophotographic apparatus on the assumption that the photosensitive member is scrubbed and polished in the course of operation and a technique of providing the photosensitive member with a measure for making it to become polished evenly to show a prolonged service life.
However, most of the proposed techniques for improving the effect of removing foreign objects consist in increasing the extent of abutment or intrusion (=deformation) of the cleaning member or the relative speed of the cleaning brush or the cleaning roller and the photosensitive member in order to increase the frictional force.
Then, as a result, the surface of the photosensitive member becomes abraded to baffle the attempt of prolonging the service life thereof.
Additionally, the cleaning blade can become chipped and the cleaning roller comes to show scars as the photosensitive member and the cleaning unit are subjected to such a heavy load. All in all, such measures come to apply an increased load onto the image-forming apparatus comprising the photosensitive member and the cleaning unit.
If such a chipped or scarred profile is not apparent, the affected member may show a change of profile that adversely affects the cleaning performance of the cleaning unit.
On the other hand, while a technique of controlling the cleaning operation by means of a magnetic material or a bias can improve the cleaning feasibility without increasing friction, some of the substances remaining on the surface of the photosensitive member may not be affected by magnetic force or Coulomb's electrostatic attractive force if such substances are for example non-magnetic.
Additionally, such a technique requires the use of a permanent magnet or an electromagnet or a power source to baffle the attempt of reducing the size and cost of the apparatus.
Thus, it is vital to clear the above problems in order to manufacture a down-sized maintenance-free electrophotographic apparatus at low cost that can maintain its cleaning feasibility in a stable manner for a prolonged period of time.
While such an apparatus should have an improved configuration, it may be indispensably necessary to improve the controllability of the effect of cleaning the surface of the photosensitive member in order to realize such an apparatus.
In other words, in order to improve the quality of the image produced by such an apparatus, the effect of cleaning the surface of the photosensitive member has to be rigorously controlled by controlling the adhesion of foreign objects and toner to the surface of the photosensitive member by means of a cleaning unit.
Japanese Patent Applications Laid-Open Nos. 60-22131, 60-22132 and 1-269945 and Japanese Patent Publication No. 4-62579 disclose techniques of defining the condition of the uppermost surface of a photosensitive member by way of the angle of contact with pure water, although none of these patent documents satisfactorily describes the correlation of the adhesion property or wettability with foreign objects such as toner with the cleaning feasibility.
It is highly desirable that the cleaning feasibility can be measured in a simple manner and the results obtained by the measurement are used to define an optimal combination of the photosensitive member and toner in order to make the electrophotographic apparatus stably produce high quality images.
Such an arrangement will be particularly effective and beneficial for reducing the servicing frequency to small electrophotographic apparatus that are to be popularly used such as laser printers, small copying machines and facsimile machines.